Conventional alkaline electrochemical cells have an anode comprising zinc and a cathode comprising manganese dioxide. The cell is typically formed of a cylindrical casing. The casing is initially formed with an enlarged open end and opposing closed end. After the cell contents are supplied, an end cap with insulating plug is inserted into the open end. The cell is closed by crimping the casing edge over an edge of the insulating plug and radially compressing the casing around the insulating plug to provide a tight seal. A portion of the cell casing at the closed end forms the positive terminal. The cell casing may also be in the form of a button cell or have a flat housing, for example, of rectangular or prismatic shape.
Primary alkaline electrochemical cells typically include a zinc anode active material, an alkaline electrolyte, a manganese dioxide cathode active material, and an electrolyte permeable separator film, typically of cellulose or cellulosic and polyvinylalcohol fibers. The term anode active material or cathode active material as used herein shall mean material within the anode or cathode, respectively, which undergoes useful electrochemical reaction upon cell discharge. The anode active material can include for example, zinc particles admixed with conventional gelling agents, such as sodium carboxymethyl cellulose or the sodium salt of an acrylic acid copolymer, and an electrolyte. The gelling agent serves to suspend the zinc particles and to maintain them in contact with one another. Typically, a conductive metal nail inserted into the anode active material serves as the anode current collector, which is electrically connected to the negative terminal end cap.
The electrolyte can be an aqueous solution of an alkali metal hydroxide for example, potassium hydroxide, sodium hydroxide or lithium hydroxide. An electrolyte comprising an aqueous solution of potassium hydroxide is more conventionally employed. The cathode typically includes particulate manganese dioxide as the electrochemically active material admixed with an electrically conductive additive, typically graphite material, to enhance electrical conductivity. Optionally, small amounts of polymeric binders, for example polyethylene binder and other additives, such as titanium-containing compounds can be added to the cathode.
The manganese dioxide used in the cathode is preferably electrolytic manganese dioxide (EMD) which is made by direct electrolysis of a bath of manganese sulfate and sulfuric acid. The EMD is desirable since it has a high density and high purity. The electrical conductivity of EMD is fairly low. An electrically conductive material is added to the cathode mixture to improve the electric conductivity between individual manganese dioxide particles. Such electrically conductive additive also improves electric conductivity between the manganese dioxide particles and the cell housing, which also serves as cathode current collector. Suitable electrically conductive additives can include, for example, conductive carbon powders, such as carbon blacks, including acetylene blacks, flaky crystalline natural graphite, flaky crystalline synthetic graphite, including expanded or exfoliated graphite. The resistivity of graphites such as flaky natural or expanded graphites can typically be between about 3×10−3 ohm-cm and 4×10−3 ohm-cm.
It is desirable for a primary alkaline battery to have a high discharge capacity (i.e., long service life) and be capable of powering flashlights, radios, portable audio players and other electronic devises operating at running voltage between about 0.8 and 1.5 Volts. It is desirable for alkaline cells to be suitable for higher power application, e.g. between about 500 and 1000 mAmp. However, there is also a need for special purpose alkaline cells to power electronic devices such as LED calculators, radios, and some electronic games, which operate at lower drain rates, for example, between about 1 and 500 mAmp. Since commercial cell sizes have been fixed, it is known that the useful service life of a cell can be enhanced by packing greater amounts of the electrode active materials into the cell. However, such approach has practical limitations such as, for example, if the electrode active material is packed too densely in the cell, the ionic conductivity can be reduced, in turn reducing service life. Other deleterious effects such as cell polarization can occur as well. Polarization limits the mobility of ions within both the electrolyte and the electrodes, which in turn degrades cell performance and service life. Although the amount of active material included in the cathode typically can be increased by decreasing the amount of non-electrochemically active materials such as polymeric binder or conductive additive, a sufficient quantity of conductive additive must be maintained to ensure an adequate level of bulk conductivity in the cathode. Thus, the total active cathode material is effectively limited by the amount of conductive additive required to provide an adequate level of conductivity.
Although such alkaline cells are in widespread commercial use the aqueous electroyte, typically comprising an aqueous solution of potassium hydroxide, requires that the cell be tightly sealed to prevent leakage of the aqueous electrolyte therefrom. The alkaline cells typically produces gassing during discharge or storage which can raise the internal pressure of the cell to elevated pressures which can reach relatively high levels, for example, of between about 600 and 1500 psia. At such internal gas pressures the casing edge at the open end of the cell casing must be tightly crimped around special end cap sealing assemblies in order to close the cell and prevent the aqueous electrolyte from leaking therefrom. The end cap assembly which typically employs an insulating disk or plug (insulating grommet) and a radially compressible end cap, or additional support disk between end cap and insulating disk, is designed to withstand high radial crimping forces necessary to provide a tight seal. Such end cap assemblies while providing a tight seal to prevent leakage of the aqueous electrolyte, nevertheless consume a significant amount of the cell's internal volume thereby reducing the amount of useable volume for anode and cathode active materials.
It would be desirable to replace the conventional alkaline aqueous electrolyte in such cells with a polymer electrolyte system which is non flowable or at least more viscous and less flowable and requires less free water than the conventional aqueous potassium hydroxide electrolyte. It would be even more desirable if the polymer electrolyte system contained little, if any free water. Such electrolyte has the distinct advantage that it would significantly reduce the sealing requirements of the cell. Such polymer electrolyte would thus markedly reduce the thickness of the end cap assembly. It could eliminate the need for conventional end cap assemblies which normally employs additional components, such as radially compressible support disks, to provide a tight seal preventing leakage of the aqueous electrolyte. In turn the amount of the cell's internal volume available for anode and cathode active materials would be increased.
It would be desirable to offer the consumer alkaline cells with a prismatic form factor that could better fit within new thin consumer products, such as cell phones, PDAs, etc. A polyelectrolyte would offer a number of advantages to a prismatic form factor alkaline cell. Prismatic cells can contain much less gas pressure without bulging than can cylindrical cells. Therefore, prismatic cells require a gas vent, and it is clearly easier to make a leakage proof gas vent if the electrolyte is not a liquid and cannot flow. The ability to fabricate thin self-supporting polyelectrolyte films and the adhesive nature of the polyelectrolyte also opens the possibility of high-speed, continuous, low-cost cell assembly by a lamination process.